Presently, the installation of an extended room on the siding of a home requires a channel over the siding in order to provide a smooth surface to mount a wall panel to the siding. Typically, the channel is U-shaped defining a groove to engage the wall panel, and having a vertical side surface opposing the groove. Sponge rubber is normally utilized to seal the vertical side surface of the channel to the siding. Unfortunately, sponge rubber does not properly fill gaps between the vertical side surface of the channel and the butt joints of the siding. Consequently, wall panels mounted to the siding via the channel may not be stable. In addition, the appearance of the extended room may not be pleasing. What is therefore needed is a device that stably and aesthetically mounts a wall panel to siding.